1. Field of the Invention
The present invention relates generally to invasive biomedical ultrasonic imaging devices of the pulse-echo type.
2. Description of the Prior Art
Heretofore, real time remote examination of the internal structure of a biological organ, vessel, or tissue has been limited to the use of x-ray techniques, external ultrasonic imaging probes, implantable "transmission mode" ultrasonic imaging devices, and imaging apparatus of the transcutaneously hard-wired variety.
Imaging by x-ray has two significant limitations. X-rays are known to be harmful to both humans and animals. They should therefore be used with caution and for only short time intervals. Low energy ultrasound, on the other hand, has not been found to cause harm to biological organs, vessels or tissues. For this reason, it is preferred to use x-rays for long periods of real-time imaging. Another limitation of x-rays is its inability to image low density structures without employing special techniques. Ultrasound is able to image certain x-ray transparent objects. For example, unlike x-rays, ultrasound is able to image the thin mitral valve within the heart.
External ultrasonic imaging probes have been discussed in the paper "A Portable Battery Powered Ultrasonoscope," R. D. Lee et al. 25th ACEMB, October 1 - 5, 1972. These external imaging devices suffer from the disadvantage that in order to image the heart, they must transmit and receive ultrasonic impulses through the space between the ribs. As a result, certain portions of the heart cannot be seen since the ribs and sternum, as well as the lungs, obstruct the beam path of the external probe. Also there is some attenuation of the signal in the tissue between the transducer and the heart wall. In animal research it is very difficult to maintain beam alignment of an external probe over long periods. It is much more satisfactory to have the probe located at the actual site of interest.
An implantable ultrasonic transmission mode device has been discussed in the paper, "Miniature Implantable Sonomicrometer System," R. Lee and H. Sandler, J. of Applied Physiology, Vol. 28, No. 1, Jan. 1970. This device, due to its mode of operation, is only suitable for measuring the dynamic wallto-wall cardiac dimension; detailed intervening structures are not visible. The device is powered by batteries, batteries which are necessarily small so that they can be implanted in a patient. Accordingly, this device has a short operational period.
The last technique involves attaching an ultrasonic transducer to the organ wall and hard-wiring the lead out through the skin to external processing instrumentation. This method enables the imaging of all sections of the heart; however, there is a high risk of an infection occurring at the site where the wires exit from the skin.